Reversing climate change even more difficult than it might sound

A thousand years might not be enough time to completely undo all the changes.

It’s often said that teenagers feel invincible. Even if they were to get hurt, a little medical treatment would quickly have them feeling right as rain. It only takes one significant injury to disabuse a person of that notion, one example that some part of their body will never quite be the same again. An injury can serve as a constant reminder that being a bit more careful would be prudent.

That, if you haven't guessed, is a metaphor for our approach to climate change. We're acting a bit like a "what, me worry?" teenager when doing damage that can be lasting. Even if humanity were to completely stop emitting carbon dioxide, it would take thousands of years for natural processes to bring the concentration in the atmosphere back down close to where it was before the industrial revolution began. But surely we’ll develop the technology to augment those processes by artificially removing CO2 long before then, right?

Well, maybe, but there are complications. One problem is that the climate system is sluggish and builds a sort of momentum that limits what intervention can accomplish. Another is that the rate of temperature change determines the impact it has on ecosystems (as well as our own ability to adapt). We’d want to limit the rate at which we lowered the concentration of CO2 and cooled the planet, or we'd do as much damage as we did while driving it up.

A recent study by Andrew MacDougall at the University of Victoria explores what that cooling might look like by using a climate model and a simple scenario—take the greenhouse gases out of the atmosphere at the same rate we put them up there. The goal was to find out whether the model could return to a pre-industrial climate by the year 3000, the end of thismillennium.

MacDougall used the same four scenarios for future emissions as the latest Intergovernmental Panel on Climate Change report. But when CO2 hit its peak in those scenarios, he made the future a mirror image of the past, bringing CO2 back down. Land use changes (like deforestation and the expansion of cropland), and other greenhouse gases were made to reverse course as well.

Depending on which scenario you look at, the planet remains 0.1 to 0.7°C warmer than pre-industrial times in the year 3000.

In a low emissions scenario, for example, the concentration of CO2 tops out at a little over 500 parts per million in the year 2130. (The concentration today is nearly 400 parts per million, and it's currently increasing about 2 parts per million each year.) The global thermometer hits 2.8°C above the preindustrial average a couple decades later before beginning to fall. Sea level reaches its high point in 2250 at about 0.7 meters above preindustrial levels. But by the year 3000, it has only fallen to about 0.3 meters. Temperature, too, is still 0.3 °C above pre-industrial at that time, though it reaches a point close to that by 2500.

Ocean pH has already dropped by about 0.1 pH units as the ocean absorbs CO2, it decreases another 0.2 units in the model before recovering by 2500. That may not sound like much, but pH is a logarithmic scale, making a decrease of 0.3 units nearly a doubling of acidity.

This outcome results from no small amount of effort. The CO2 removal technology in this scenario needs to get switched on in 2130 and kept operating continuously (gradually tapering off) until the year 2630. It goes without saying that action on this kind of scale would come with a pretty hefty price tag. And remember, this is the low emissions scenario.

Because warming causes additional carbon to be released into the atmosphere, such as is the case with thawing permafrost, we’ll actually have to remove significantly more CO2 from the atmosphere than we put there by burning fossil fuels. In fact, we'd have to remove roughly 40 percent more in the model simulations.

There’s also the uncertain behavior of ice sheets to consider. In the high emissions scenario, the simulated Greenland Ice Sheet crossed a tipping point, losing enough ice to raise sea level about 0.7 meters more than the other scenarios (1.4 m total). By the end of the millennium, it had only recovered enough to bring sea level down about 0.2 meters from that high point. It takes a long time to rebuild an ice sheet.

The study concludes, “These results suggest that even with monumental effort to remove CO2 from the atmosphere, humanity will be living with the consequences of fossil fuel emissions for a very long time.” Solutions that limit the amount of CO2 we end up emitting this century are investments. Not only do they obviously pay off in terms of warming avoided, but they also lessen the burden on future generations should they try to clean up the atmosphere they inherit.